The present invention generally relates to testing of integrated circuit (IC) devices, and more particularly to the design of pogo-pins used in socket test probes, contactors and the like for achieving reliable electrical contact with the solder ball contacts of an IC device. The invention has particular application in the testing of or interfacing with ball grid array (BGA) devices wherein electrical contact must be achieved with an array of densely packed solder ball contacts having spherical or hemispherical contact surfaces.
When testing IC devices a reliable electrical interface must be achieved between the test probes and the IC device in order to produce reliable test results. Testing devices typically utilize an array of individual conductive probes called "pogo pins", which provide a compliant contact array to electrically interface with the I/O contacts of the IC device. Generally, the tips of the pogo pins are designed to achieve efficient electrical contact between the pogo pin tip and the tested or contacted device, and to minimize the introduction of contact resistance in the signal path. However, little or no regard is given to the possible distortive effects of the probe on the I/O contacts themselves.
As IC packaging has become smaller, and I/O densities higher, ball grid arrays (BGAs) have emerged as an alternative IC packaging for integrated circuit devices having extended pin contacts. In BGA devices, the I/O contacts are provided in the form of an array of densely packed solder balls, which permit the device to be surface mounted to a circuit board. In addition to the efficiencies of surface mounting, BGAs permit smaller packaging for higher circuit densities and greater functionality with reliable electrical and thermal performance. On the other hand, with smaller BGA sizes come problems with testing. Specifically, in very small devices, known as micro-BGAs, solder balls are very small and have greatly reduced surface areas. In these devices the deformation of the solder balls caused by the probe contact is relatively large compared to the overall size of the balls, and this can detrimentally affect both the inspection and assembly of the device onto a printed circuit board. For example, excessive deformation of the solder balls can cause devices to be rejected during quality control procedures which may employ laser inspection techniques. Also, surface deformations can lead to problems in the integrity of the solder joints between the micro-BGA and the circuit board when the micro-BGA is mounted to the board.
Heretofore, probes used in the testing of BGA and micro-BGA devices have had probe tips with single or multiple points which dig into and around the apex of the BGA's solder balls. Such probe tips generally leave deep incision marks in the solder ball contacts and also deform the solder balls from their original shape. Furthermore, with such tip designs separation of the solder balls from the probe tips requires a degree of force that is generally undesirable. One reason for this is the relatively deep penetration of the points of the probe tips into the solder balls which causes the solder balls and probe tips to come into contact over a substantial surface area. Also, such probe tips are typically coated or plated with metallurgy, such as gold, that readily fuses or migrates into the soft solder material of a solder ball contact.
The present invention overcomes the problems associated with surface deformation of the solder ball contacts of a BGA device caused when the BGA's solder ball contacts are contacted by external conductive probes. The present invention further overcomes problems associated with the release force necessary to separate the BGA devices from external probes used to make electrical contact with such devices.